The present invention relates to the manufacture of partially aromatic polyamides. In particular, the present invention relates to a process for making a partially aromatic polyamide from an aliphatic diamine and the dialkyl ester of an aromatic dicarboxylic acid.
Partially aromatic polyamides consist of aromatic dicarboxylic acid and aliphatic diamine monomer units. Such polyamides are generally characterized by high melting points, high glass transition temperatures, low moisture absorption and, unlike aliphatic polyamides such as nylon 6 and nylon 66, good dimensional stability under moist conditions. The combination of high temperature and dimensional stability render partially aromatic polyamides particularly suitable for use in electronics, engineering plastics, films and fibres.
Unfortunately, however, the majority of partially aromatic polyamides are difficult to manufacture using the conventional melt polycondensation process that is successfully used in the manufacture of aliphatic polyamides. These processes generally involve admixing a dicarboxylic acid and a diamine to form a salt in aqueous solution. The salt is heated to a temperature that is higher than the melting point of the polyamide being formed but that does not result in excessive thermal degradation of the desired polyamide. U.S. Pat. No. 5,502,155 to Ng, issued Mar. 26, 1996, does, however, describe such a process for making partially aromatic polyamides. The process involves heating an admixture of an aromatic dicarboxylic acid and an aliphatic diamine to a temperature of at least 270xc2x0 C. at a pressure of at least 1.2 MPa in the presence of a monocarboxylic acid such as formic acid. Water is added incrementally during heating. This process is particularly suitable for making polyamides from 2,6 naphthalene dicarboxylic acid, and cannot be universally applied to all partially aromatic polyamides.
Partially aromatic polyamides are characterized by melting points of at least about 275xc2x0 C. and in some instances, melting points of greater than 300xc2x0 C. Such high melting points generally result in significant thermal degradation during the synthesis of the desired polyamide. Moreover, branching formation side reactions compete with the polymerization reaction at the high temperatures required to maintain the partially aromatic polyamide in the form of a melt. These side reactions lead to serious melt viscosity build-up as the molecular weight of the polyamide increases. The viscous melt traps gaseous condensate within the polymeric molecule. This causes voids to form in the polymer which make subsequent processing of the polyamide difficult.
Processes have been developed which resolve some problems associated with making partially aromatic polyamides by conventional means. For example. Wittbecker and Morgan (Journal of Polymer Science, 40:280 (1959)) describe an interfacial polycondensation process in which an acid chloride, such as a dicarboxylic acid chloride, is reacted with a compound containing an active hydrogen atom (xe2x80x94OH, xe2x80x94NH and xe2x80x94SH) near the interface of the two phases of a heterogeneous liquid system, for example benzene in water. Yamazaki et al. (Journal of Polymer Science, 13:1373-1380 (1975)) describe a low-temperature method of reacting the phosphite and phosphonate salts of aromatic diamines and aliphatic dicarboxylic acids in a pyridine solution in the presence of metal salts such as LiCl or CaCl2. However, both of these methods are too costly for practical use on a commercial scale, and nevertheless, would be difficult to conform to continuous operations.
U.S. Pat. No. 3,642,710 to Keen, which issued Feb. 15, 1972, describes a process for making high molecular weight polyamides such as polydodecamethylene terephthalamide, which is a partially aromatic polyamide, at a decreased temperature. Specifically, the reactant, dodecamethylene diammonium terephthalate, is heated at a temperature of about 255-275xc2x0 C., in the presence of a viscosity stabilizer, a reagent capable of controlling the molecular weight of the polyamide when the polymerization reaction attains equilibrium. In this process, the reaction mass remains solid and can subsequently be melt spun into filaments.
U.S. Pat. No. 3.917,561 to Chapman and Pickett, which issued Nov. 4, 1975, teaches another process for making polydodecamethylene terephthalamide in which a cation-exchange treated dodecamethylene diammonium terephthalate salt is melt polymerized in the presence of a sterically hindered phenol, benzenephosphinic acid, copper acetate in combination with an alkali metal halide or a mixture of any of these additives. In this process, the additives cooperate with the cation-exchange treated salt to provide a melt-stable polyamide. Both this method and the method of Keen described above undesirably involve the step of forming a salt from the diacid and diamine reactants and the use of special additives such as stabilizers. The Chapman and Pickett method further requires the costly step of salt purification by ion-exchange.
Processes for making high molecular weight polyamides at lower temperatures have also been developed. In this regard U.S. Pat. No. 4,131,712 to Sprauer, which issued Dec. 26, 1978, describes a process in which a dicarboxylic acid-rich (diacid-rich) component and a diamine-rich component are combined with heating in the absence of water to form a polyamide. The diacid- and diamine-rich components each have melting points which are depressed in comparison to the pure diacid and diamine compounds, advantageously allowing the polymerization to be conducted at a lower temperature, and thereby minimizing thermal degradation. Such a process cannot be used in the manufacture of partially aromatic polyamides for a number of reasons. At the outset, the aromatic dicarboxylic acid reactants used to synthesize partially aromatic polyamides have an extremely high melting point and often cannot be melted without themselves being thermally degraded. Further, these reactant mixtures are not stable under anhydrous conditions, conditions which are central to the Sprauer process.
It has now been found that partially aromatic polyamides can be manufactured by combining an aromatic dicarboxylic acid component, at least a portion of which is in the form of an alkylated ester, with a diamine component in the presence of water. Esterification of the dicarboxylic acid advantageously lowers its melting point to a temperature that allows melting of the acid while avoiding, or at least minimizing, thermal degradation thereof. Admixture of the dicarboxylic acid component and the diamine component in the form of a melt is thereby facilitated. Further, the partially aromatic polyamide formed by these reactants likewise contains the alkyl sidechains and these sidechains function also to depress the melting point of the polyamide, yielding a polyamide that is more readily processed than the corresponding polyamide that lacks such alkyl sidechains.
Accordingly, the present invention provides a process for making a partially aromatic polyamide from at least one aromatic dicarboxylic acid component and at least one aliphatic diamine component comprising a diamine having from 6-12 carbon atoms, wherein 20-100% by weight of the dicarboxylic acid in said acid component is in the form of an alkylated ester, said process comprising the steps of:
(a) admixing non-stoichiometric amounts of the acid component with the diamine component in the presence of water;
(b) heating the admixture to a temperature at which it forms a melt while discharging therefrom volatile matter;
(c) further heating the admixture to a temperature above the melting point of the partially aromatic polyamide to form a polyamide oligomer;
(d) adding sufficient amounts of an aliphatic diamine having from 6-12 carbon atoms, or an aromatic dicarboxylic acid, at least a portion of which is in the form of an alkylated ester, so that the total amounts of the acid component and diamine component in the admixture of step (c) are approximately stoiciometric;
(e) heating the admixture of step (d) to a temperature at which it forms a melt while discharging therefrom volatile matter; and
(f) further heating the admixture to a temperature above the melting point of the stoichiometrically-balanced partially aromatic polyamide to form the polyamide.
In a further aspect of the present invention, there is provided a partially aromatic polyamide formed from at least one aromatic dicarboxylic acid component and at least one aliphatic diamine component, wherein 20-100% by weight of the dicarboxylic acid of said acid component is in the form of an alkylated ester and said diamine component comprises a diamine having from 6-12 carbon atoms, said polyamide comprising from 1-100% on a molar basis of N-alkylated amide and amine groups.
A novel process for making partially aromatic polyamides is provided in which an aromatic dicarboxylic acid component, at least a portion of which comprises an alkylated ester of the aromatic dicarboxylic acid, is combined with a diamine component comprising a diamine having from 6-12 carbon atoms.
The aromatic dicarboxylic acid component suitable for use in the present process may be selected from the group comprising an aromatic dicarboxylic acid, at least a portion of which is in the form of an alkylated ester; and a dicarboxylic acid oligomer comprising a non-stoichiometric amount of an aromatic dicarboxylic acid, at least a portion of which is in the form of an alkylated ester, and an aliphatic diamine having from 6-12 carbon atoms in which the balance of the acid oligomer comprises the dicarboxylic acid.
Suitable aromatic dicarboxylic acids for use as the dicarboxylic acid component in the present process include terephthalic acid, isophthalic acid and naphthalene dicarboxylic acids. Mixtures of these acids can also be used. In accordance with the present invention, at least a portion of the dicarboxylic acid must be in the form of an alkylated ester. In this regard, preferably at least 20% by weight of the acid is in the form of an alkylated ester. More preferably, at least 40-75% by weight of the acid is in the form of an alkylated ester, and most preferably, essentially 100% by weight of the acid is in the form of an alkylated ester.
Alkylation of the dicarboxylic acid to form an alkylated ester is carried out using processes well-known in the art, for example, the esterification of an acid by an alcohol as described in xe2x80x9cAdvanced Organic Chemistry Reactions, Mechanisms and Structures (J. March. McGraw Hill, 1968, p.320). The dicarboxylic acid may be alkylated with groups containing from 1-4 carbon atoms. Preferably, the dicarboxylic acid is alkylated with groups containing 1-2 carbon atoms, i.e. methyl and ethyl groups. A particularly preferred alkylated ester in accordance with the present invention is a dialkylated ester of a dicarboxylic acid.
Use of the aromatic dicarboxylic acid, either partially or wholly in the form of an alkylated ester, in the present process provides a number of advantages not realized in prior art processes for making partially aromatic polyamides. At the outset, the melting point of the alkylated ester form of aromatic dicarboxylic acids is substantially lower than the melting point of the dicarboxylic acid itself. The profound differences in melting point between an aromatic dicarboxylic acid and its alkylated ester can be illustrated by terephthalic acid and its dialkylated ester, dimethyl terephthalate. The melting point of terephthalic acid is greater than 400xc2x0 C., while the melting point of dimethyl terephthalate is about 140xc2x0 C. In some cases, the melting points of aromatic dicarboxylic acids are so high that the acid is thermally degraded before the melting point is attained. Thus, the presence of the acid in alklyated ester form functions advantageously to depress the overall melting point of the acid. Depression of the dicarboxylic acid melting point is beneficial because it decreases the amount of time that the reactants are exposed to elevated temperatures, for example temperatures exceeding the melting point of the desired partially aromatic polyamides which typically range from 260xc2x0-320xc2x0 C., the temperature at which the final step in the present process is conducted. This decrease in the use of elevated temperatures is significant in minimizing thermal degradation of reactants and product during the process. It also plays a significant role in minimizing branching formation side reactions which result in the formation of voids in the polyamide that make subsequent processing of the polyamide difficult. A further advantage of using the alkylated ester of dicarboxylic acids in the process of making the partially aromatic polyamides is that the aliphatic diamine reactants have good solubility in molten aromatic alkylated ester.
Oligomers suitable for use as the dicarboxylic acid component of the present process are formed from a suitable aromatic dicarboxylic acid as set out above, at least a portion of which is in the form of an alkylated ester, and an aliphatic diamine, either linear or branched, having from 6-12 carbon atoms. In this regard, suitable linear diamines include hexamethylene diamine, heptamethylene diamine, octamethylene diamine, nonamethylene diamine, decamethylene diamine, undecamethylene diamine and dodecamethylene diamine, while suitable branched diamines include 2-methyl pentamethylene diamine, 3-methyl pentamethylene diamine, n-methyl-1,6-hexamethylene diamine wherein n is 2 or 3, n-methyl-1,7-heptamethylene diamine wherein n is 2, 3 or 4, n-methyl-1,8-octamethylene diamine wherein n is 2, 3 or 4, and n-methyl-1,12-dodecamethylene diamine wherein n is 2, 3, 4, 5 or 6. Mixtures of suitable diamines are also appropriate for use in preparing the acid oligomer.
The acid oligomer is prepared by combining a non-stoichiometric amount of the dicarboxylic acid with a suitable diamine such that the oligomer comprises excess dicarboxylic acid, in the form of both acid and alkylated ester. The oligomer may be formed by melting the dicarboxylic acid together with the diamine. In an alternative, the oligomer may be made by physically mixing the acid and diamine together. In another alternative, the oligomer may be made by admixture of the acid and diamine in an aqueous solution to form a salt. Regardless of the method used to prepare the oligomer, the reactants must be heated to a temperature that is greater than the melting point of the oligomer but less than the melting of the polyamide that the oligomer will be used to make. During heating, water is evolved from the reaction and is vented from the process. After a substantial portion of the water has been removed from the reaction mixture, the process of making the oligomer is complete.
The diamine component suitable for use in the present process may be selected from the group comprising an aliphatic diamine having from 6-12 carbon atoms, such as those set out above; and a diamine oligomer comprising a non-stoichiometric amount of the diamine and an aromatic dicarboxylic acid, at least a portion of which is in the form of an alkylated ester, in which the balance of the diamine oligomer comprises diamine. The diamine oligomer may be made using methods similar to those used to make the acid oligomer as set out above.
The acid and diamine oligomer components are relatively low melting components when in a monomeric form. For instance, the oligomer formed by mixing the dialkyl ester, dimethyl terephthalate, with decamethylene diamine has a melting point in the range of from about 110xc2x0 C. to about 140xc2x0 C. over the range of oligomer compositions formed by admixture of differing proportions of dialkyl ester and diamine. The melting point of the oligomer formed by mixing dimethyl terephthalate and a diamine that is a mixture (1:1) of hexarmethylene diamine and 2-methyl pentamethylene diamine ranges from about 25xc2x0 C. to about 140xc2x0 C. over the range of oligomer compositions that can be formed.
The process of the present invention includes admixture of the dicarboxylic acid component with the diamine component in the presence of water. The amount of each component added to the admixture is preferably a substantially stoichiometric amount, taking into account the total amount of each component in the reactant mixture, whether bound or free. In particular, the acid and diamine oligomers may contain both bound and free dicarboxylic acid and diamine and this should be taken into consideration. As one of skill in the art will appreciate, equimolar amounts of dicarboxylic acid and diamine will desirably yield a polyamide having the highest molecular weight. There may be some instances in which stoichiometric amounts of the components is not desirable. For example, to provide a polyamide with particular dyeability characteristics, it may be appropriate to use a small excess of dicarboxylic acid or diamine to produce a polyamide having either excess acid or diamine ends.
The amount of water required in the admixture is an amount sufficient to maintain the reaction in a stable condition. In this regard, the water should be at a level that eliminates, or at least minimizes, flashing or instantaneous evaporation of the reactants, as well as any other unstable conditions that may occur in the reactor. Preferably, the amount of water admixed with the components is at least 5% by weight of the reaction mixture, more preferably at least 10% by weight of the reaction mixture and most preferably at least 20% by weight of the reaction mixture.
The present process may optionally be conducted in the presence of a catalyst in order to accelerate reaction time. Suitable catalysts include phosphorous-containing compounds such as phosphinic acid and/or the sodium or potassium salts thereof, hypophosphorous acid, sodium hypophosphite, phosphoric acid and the like. In this regard, an appropriate amount of catalyst that could be added to the reaction mixture would be from about 0.05-2% by weight. A preferred amount of catalyst to be added to the mixture would be in the range of about 0.10-0.20% by weight.
Following admixture, the reactants are heated, generally in a reactor of the type typically used in the polymerization of polyamides, for example a stainless steel autoclave, in a controlled manner to a temperature at which the admixture forms a melt. The temperature will, of course, depend on the characteristics of the reactants used, but will preferably be a temperature at which thermal degradation and side branching formation problems are minimized as noted above. Volatile matter, and more particularly, nonessential volatile matter is discharged from the reactor while maintaining stable reaction conditions.
Once a substantial amount of the volatile matter has been vented, the admixture is further heated to a temperature above the melting point of the polyamide being formed so as to obtain a polyamide of a desired inherent viscosity, e.g. in the range of about 0.4-1.5 dL/g, and preferably in the range of about 0.6-1.0 dL/g. Again, the temperature will vary with the polyamide being formed, but will generally be in the range of 260xc2x0-320xc2x0 C. The temperature is maintained for a sufficient period of time to drive polymerization of the components substantially to completion. As the reaction progresses, the temperature may have to be increased to avoid the separation of solids. This increase in temperature will vary but will generally be in the range of about 10-50xc2x0 C., and preferably in the range of about 20-30xc2x0 C. above the final melting temperature of the polyamide being formed.
The admixture may be heated under pressure, or heated at atmospheric pressure. The process is preferably conducted under pressure, preferably a pressure of between about 1 MPa and 2 MPa. more preferably a pressure of at least about 1.3 MPa (1300 kPa), and most preferably a pressure of at least about 1.7 MPa (1700 kPa). The pressure may be maintained constant throughout the process, or alternatively, once the elevated temperature of polymerization is attained, the pressure may be reduced so as to subject the polyamide to a xe2x80x9cvacuum finishingxe2x80x9d step in which the pressure in the reactor is reduced to less than atmospheric pressure, preferably by about 50-60 kPa, on application of a vacuum. This step serves to increase the molecular weight of the resulting polyamide. During pressure reduction, the pressure should be reduced in a manner that minimizes or avoids excessive foaming of the reaction mixture in the reactor. In this regard, anti-foam agents, which include polyethers such as Carbowax(trademark), are preferably added to reduce the amount of foaming.
In a further aspect of the present invention, there is provided a partially aromatic polyamide formed from at least one aromatic dicarboxylic acid component and at least one aliphatic diamine component comprising a diamine having from 6-12 carbon atoms, wherein 20-100% by weight of the dicarboxylic acid of said acid component is in the form of an alkylated ester, said polyamide comprising from 1-100%, preferably at least about 15%, more preferably at least about 20% and most preferably at least about 50%, on a molar basis of N-alkylated amide or amine groups.
The polyamides of the present invention may be amorphous or partially crystalline polyamides having a beat of fusion as measured by differential scanning calorimetry (DSC) of greater than 17 J/g. The present polyamides preferably have an inherent viscosity in the range of about 0.4-1.5 dL/g, and particularly in the range of about 0.6-1.0 dL/g. The polymers generally have melting points greater than 260xc2x0 C., preferably in the range of about 280-320xc2x0 C. and most preferably in the range of about 290-310xc2x0 C. Advantageously, the melting point of an alkylated polyamide prepared in accordance with the process of the present invention is generally at least 5xc2x0 C. lower than the melting point of the corresponding non-alkylated polyamide.
The polyamides may be used in the manufacture of products using melt processing techniques, especially products intended for use at elevated temperatures or products in which retention of properties at elevated temperatures is required. For example, the polyamides may be formed into articles using injection moulding technology, for example, valves tanks, containers, washers and the like, parts for automotive end-uses, particularly those requiring resistance to temperatures of 260xc2x0 C. or more, and articles where retention of mechanical properties under the influence of heat, moisture, hydrocarbons, alcohols including so-called gasohol, and the like are important articles such as, for example, retortable containers. Alternatively, the polymers may be spun into fibres, preferably having a tenacity of at least 1.5 g/denier and a modulus of at least 30 g/denier, for use as sewing or industrial thread where low shrinkage and elongation are important and/or retention of properties under the influence of moisture, hydrocarbons, alcohols and the like is important. The polyamides may also be formed into films or sheets having end-uses as, for example, electronic printed circuit boards, industrial packaging films, electrical insulation films, and substrates for coating. The barrier properties of the present polyamides to water and oxygen are further characteristics which may determine additional end-uses.